The mammalian kidney has a substantial capacity to excrete an alkaline load. For example, rodents given isotonic NaHCO3 in their drinking water develop only a mild metabolic alkalosis despite this huge base load. The kidney's ability to excrete large alkaline loads occurs in part through active HCO3- secretion in the cortical collecting duct (CCD) mediated by apical anion exchange in the B intercalated cell. While apical anion exchange has been characterized functionally in the CCD, the gene product, which mediates this transport process has remained an area of controversy. However the work of our laboratory and that of our collaborators has demonstrated that pendrin, an anion exchanger, localizes to the apical membrane of non-A intercalated cells in rat, human and mouse. Moreover, our laboratory has demonstrated that pendrin mediates secretion of HCO3- in the mouse CCD. Whether this transporter represents the putative apical anion exchanger remains to be tested fully. Development of knockout mice has greatly facilitated the study of the renal physiology of pendrin in vivo. Because there are no specific inhibitors of either pendrin or apical anion exchange available, knockout mice can be exploited as a biological inhibitor and used to test transport characteristics of pendrin in kidney in vitro. Moreover, balance studies using these animals will allow us to determine if these pendrin-deficient mice have a defect in excretion of an alkaline load. Thus, the present proposal will test if pendrin represents the putative anion exchanger of the CCD. Moreover, these studies will contribute to our understanding of the physiological role of this transporter. To answer these questions our laboratory and that of our collaborators will employ balance studies, tubules perfused in vitro, real-time quantitative RT-PCR, immunogold cytochemistry and immunohistochemistry studies in wild type and pendrin-deficient mice. The Specific Aims of the project are: 1. To determine the transport characteristics of pendrin in native renal tissue, 2. To determine if pendrin is upregulated in kidney during metabolic alkalosis, 3. To determine the renal phenotype of pendrin and 4. To determine the localization of pendrin in kidney. These studies will contribute to our understanding of how the mammalian kidney excretes an alkaline load.